This invention pertains to methods and apparatus for monitoring and deriving indications of the speed at which a pipe string travels while such pipe string is lowered into or pulled out of a well bore during a well drilling operation.
In a rotary well drilling operation, a drilling bit at the end of a drill string or pipe is rotated to cut into the earth formations. The drill string or string of pipe is made up of pipe joints, usually about 30 feet in length, which are coupled to one another by threaded tool joints. As the well is drilled, additional pipe joints are added to the string of pipe.
From time to time during a rotary drilling operation, the drill string or pipe is removed, for example, to change the bit or to perform another operation, such as obtaining a log of the formations. Thereafter, the pipe is returned to continue drilling or to run a string of casing into the hole. The process of removing and reinserting a string of pipe is called "tripping".
In removing or reinserting a pipe string, the pipe joints are successively uncoupled or coupled as the case my be and the sections of pipe or joint sections are stacked in the drilling rig. Usually, pipe joints are interconnected lengths of two or three pipe joint sections, sometimes referred to as "doubles" and "Threbbles" or "pipe stands" when they are vertically stacked in the drilling rig. In this relationship, the pipe joints also are said to be "racked" in the derrick.
The drilling rig has a device called a rotary table which is used to rotate the drilling string. In the rotary table on the derrick floor of the rig are releasable slips which are used to releasably support the pipe string in the borehole during the tripping operation. For example, while going in with a pipe string, the slips serve to hold the upper end of the pipe string in the rotary table and prevent the pipe string from dropping into the well. A vertically movable traveling block in the rig derrick is used to bring a double or threbble pipe length or stand into a position where such a stand can be threadedly coupled to the upper end of the pipe supported in the rotary table. Upon interconnection of a pipe stand to the pipe string, the slips are released and the traveling block supports and lowers the string of pipe into the well bore until the upper end is just above the slips, whereupon the slips in the rotary table are reengaged with the pipe string. This operation is continued until the bit at the lower end of the pipe string is in drilling position. From the time that the pipe string begins its motion from a stop or rest position, it first accelerates to a "running-in" speed which is essentially a constant speed, and then it decelerates to a stopped condition.
Typically, in well drilling operations, a drilling fluid (commonly called "mud") is used where the functions and properties of the drilling fluid are intended to promote a safe and speedy drilling and completion of the well. While the pipe string is being moved into the bore hole or from a bore hole, hydraulic effects or pressure surges relative to the borehole are created which can damage the subsequent productivity of hydrocarbon-bearing formations. Excessive surge pressures can also lead to loss of drilling fluid through pressure induced fractures of the formation which can cause sticking of the drill pipe, excessive loss of mud and other complications.
When the pipe is removed from the borehole, the above described procedure is reversed in that the pipe joints are racked in the derrick as double or triple stands as they are successively uncoupled from the string of pipe. During the pipe removal operation, the motion of the pipe is first an acceleration to a constant speed and then deceleration to a stop condition. While coming out of the borehole, if the string of pipe is pulled too fast while being removed, a condition known as "swabbing" and other undesirable hydraulic effects can occur. Swabbing is a condition involving a reduction in the total hydraulic pressure in the hole to a less than normal pressure for the hydrostatic pressure of the static drilling fluid column in the well bore. An excessive reduction in hydraulic pressure can cause the well to "kick", that is, formation fluids under their in-situ pressures may enter into the drilling fluid and into the well bore. This action could cause a "blow out". In soft formations, collapse of the borehole walls can also occur because of swabbing effects.
Swab and surge pressures can be minimized by reducing the viscosity characteristics of the drilling fluid, providing adequate borehole to pipe clearances, and minimizing flow constrictions in the pipe string. These factors are considered and taken into account when planning the drilling operations for a well. While a round trip of the pipe string is being made, however, these pressures can only be controlled by driller in control of the pipe speed. Commonly, a listing or schedule with optimum velocities in terms of information such as "pull 10 stands at 85 seconds per stand" etc. is available for use by the driller. This schedule can be computed by hand or by a computer. The driller will then attempt to pull or run the pipe string at a uniform velocity by noting the total time required for moving one stand or a joint of pipe over a given distance. However, the drilling cannot give undivided attention to the pipe speed requirements because he must be attentive to the actions of his other crew members in the synchronized operation of moving a pipe string as well as the other equipment under his control.
The schedule, however, cannot always take into consideration miscellaneous factors, which sometime affect velocity, such as the amount of drag on the moving pipe, the position of the hoisting equipment and the behavior of the machinery. Moreover, even if the operation follows the schedule and average speed is within the prescribed limits, it is possible for the instantaneous speeds to be excessive and cause damage.
If the driller simply pulls or runs the pipe very slowly, the hydraulic pressure can be controlled but this is undesirable since it is costly in terms of rig time consumed and furthermore, excessive time periods without mud circulation (as when tripping) may lead to various well difficulties. It should be noted that an optimum velocity varies as a function of the amount of moving pipe in the hole, generaly, but not necessarily, decreasing with increasing lengths of moving pipe.
It should be appreciated from the foregoing that tripping the pipe is a synchronized operation of the drilling crew to move the pipe into or out of the borehole in as short a time as possible, not only to reduce costs but also to reduce the risks involved in not having the pipe in the hole where mud control can be maintained. As noted heretofore, the technology to date for the driller to determine proper run-in or run-out speeds of a string of pipe involves only rudimentary execution procedures based on the elapsed time for moving a section of pipe. A stop watch is sometimes used as the determinant for the velocity, and it will be appreciated that this can only establish average values for velocities. This technique has the very obvious disadvantage that excessive velocities may occur even though the average velocity is kept within limits and the impreciseness of the operation can unknowingly cause well damage.